"Runnin' Home to You", the song Kara sings at the wedding, is the same song Barry sings to Iris when he proposes to her at the end of "Duet (2017)". It was written by Benj Pasek and Justin Paul, who won the Oscars for Best Original Score and Best Original Song for La La Land (2016).
In the DC Universe, Earth-X was a version of Earth where World War II ended with a complete German victory in the year 1968. It was the home of the Freedom Fighters, a team of heroes that led the resistance against the Nazis, and finally overcame them with help from the Justice League and the Justice Society.
When Sara says to Alex "How's your butt?," the line wasn't actually scripted, Caity just added it and therefore, Chyler's reaction was completely natural. They had to re-film her reaction, however, as because she didn't know what to say, she laughed.
The Yalta conference, as mentioned at the end of the episode, was a forum for the leaders of the United States, Great Britain, and the Former Soviet Union (Roosevelt, Churchill, and Stalin), to resolve issues of the Post World War II era. Yalta is a resort city, in the contested region of the Crimean Peninsula, claimed by both Russia and Ukraine as of February 2014.
Mick asks Caitlin if he kidnapped her once, reference to the events of The Flash S1 episode 10 "Revenge of the Rogues" (2015).